I have a C4, with the typical 6 38DCOE side drafts. I ran across a flat spot about a year ago that occurs under load in higher gears. In third gear, at 3000 rpm, if I mash the peddle to the floor, I will get a hesitation, like a chugging up to 4000 rpm. At 4000 rpm (where the cam seems to come in) it will take off like a rocket. If I press the accelerator down more slowly, there is now issue,it just goes fine. I'm running factory Euro spec timing. Main jets were increased to 130s (which really helped - I recommend it) Idle jets are 60f8s, and the float levels are set at 5mm. I have Dual MSDs using the one set of points in each distributor as the triggers with the mechanical advance. I have the stock closed accelerator bypass valves, stock accelerator pump springs (Strongest weber made) and the stock f29 emul tubes. I bumped up the accelerator jets to .45s, from .35s thinking that I had fuel starvation issue. It does not seem to have had any effect. In trying to feel what is going on, I can't tell if opening up butterfly's too quickly in this mid rpm range is creating a fuel starvation issue, or am I hosing down the combustion with the accelerator pump. I have no back fire through the carb and no backfire out the tail pipe that I can tell. Also - I have not been able to see black smoke - Any ideas? is this normal? Thanks, Aaron
Does this happen (at the same RPM) in any other gear ? If so, it may be ignition related....? Otherwise, the carbs are starving a bit for either fuel or air ---- question is which....only way I would know how to determine that is to install a pair of aftermarket A/F sensors (one in each bank) and monitor the data while driving ---- look for "lean" or "rich" readings when you hit that condition, and experiment with carb adjustments accordingly. Alternatively, if you can find a shop with a Dyno, and a tail pipe emissions detector set-up, you could find out that way as well by running the car while monitoring. You might be able to "charm" an official at your local DEQ/EPA Testing Station into letting you take a spin on their equipment, too ---- will cost you much less than a hour of a shop's dyno time
I suspect the advance curves of your distributors do not match. Maybe one or both of the mechanisms are sticking, they usually are mismatched to some extent anyways. The elegant solution is to convert one distributor to firing all 12, and use the other distributor only for distributing to 6. This way, you only need one advance curve.
Have you measured the amount of fuel the accelerator pumps are dispensing? Just a thought since it happens only when you mash the throttle.
There is a super simple fix that's been talked about many times here. Most likely you just need to fatten up the main jets a bit more. I changed mine to 135's......Problem totally gone. Geffen
First, thanks for all the input and Ideas. I'm going to need to think through the single distributor concept. I would love to get away from two separate mechanical advance curves. Regarding the 135 main jet size - yep, that solves most of the problems. Regarding the issue I'm seeing. I have a new theory. The accelerator jets have a aluminum crush washer that seals the base to the carburetor body. Any leaks in this seal will allow gas pushed by the accelerator pump to leak around the jet and into the carburetor throat. This leakage effectively makes the accelerator pump delivery larger. This aluminum crush washer should only be used once (I did not realize this) and consequently I had taken the jets out to check sized on several occasions without replacing the washer. So, I found a statement on a Holley carb web site describing the excessive accelerator pump delivery as a "Fluttering" in acceleration and inadequate accelerator pump delivery more as a cutoff. I have experienced this "cutoff" feeling, things just kinda give up. Based on this, my thinking is more towards too much accelerator delivery. To answer one of the questions - no I have not measured it, but if I hold a carb up and actuate the throttle, it will give me two streams of gas the extend about 5 feet and run for 2 to 3 seconds - so it is lots especially considering there are 12 of these. I order new aluminum crush washers and I will use them to install the original .35 jets. The .45 jets were installed with new aluminum crush washers and acted about the same as the .35 jets with what I theorize as leaking washer seals. Hopefully this will do it. If not, I will taking a harder look at my ignition. Thanks, Aaron
95% of the carb problems in these cars end up being almost 100% problems with the ignition! Having had several of these, your carb mods, in all honesty, sound like the thing is drowning in gas! apparently not, but suspect if curves are appropriate and MATCHED 100% equally side to side at equivalent RPMs, the fuel mods require closer examination... with stock cams, these cars were overcarberated New...imo...the progression circuit shifts right in the area of complaint, so i maintain my inclination to suspect too m uch fuel and curves not spot on-a condition critical in my experience with USA cars using usa gasoline... let us know how this works out....(have you put stock jets back to se if car runs "ok"?
355s, Thanks for the post - The experienced perspective is much appreciated. I'm hoping to get my new washers today. I may get a chance to try them out on Sunday. I will report back on the "leaking washer theory".
I have the original pump jets back in, this time with new washers. It is better, so the issue is looking like a over rich condition when the accelerator is mashed to the floor under high load where RPM increase is slower between 3000 and 4000 rpm. More on this mashing of the accelerator. First gear - goes great - quick past 5000 rpm Second gear - 2500 rpm - goes great past 5000 rpm 3rd gear - 2500 rpm. this time on a hill so the RPMs do not climb very quickly - still have some fluttering between 3000 and 3800 when the peddle is mashed. If I apply the accelerator a bit more slowly, the acceleration is smooth and strong. After 3800 rpm - no issues, hard acceleration. At this point it is not clear if I want to tune any more. I'm thinking that I could put a bypass in the accelerator circuit, but I my be trying to fix something that was not really meant to be fixed. If I get a chance to drive another C4 some time, I will see if situation exists there. Regarding ignition - because the situation is so dependent on throttle application and only exists under heavy load in a narrow RPM range, it is unclear to me as to how this would relate to ignition - but am certainly open to learn new things. Thanks for listening, Aaron
Aaron, I have been following this with interest since it seems a different malady than normal carb issues. Over the years I have had a BB, 2 C/4s and a Dino that suffered from "hesitation" under gradual acceleration. Idling all were fine. And hard acceleration overcame the hesitation. In each case the main jets were upsized. Problem solved. It was explained to me that the junk we now passes as "benzina" requires larger main jets. Yours is different in that the problem occurs under hard acceleration while gradual acceleration is fine. It seems you are on the right track with your going back to original sized accel jets. Out of curiosity.......what is the look of the tail pipes? Dark gray or brown? Or a "heavy" black? Back when we had decent gas you could check your state of carb tune by the color of the exhaust. I liked to set carbs to achieve a medium gray. Nowadays it seems running richer is a necessity.
The accelerator pump jets ARE a bypass. The bigger the number, the less fuel is squirted during throttle movement. Is there a chance you are going backwards? i.e. putting in a bigger numbered accel pump jet and thinking you will get more fuel? You don't mention that ever did anything with accel pump jets one way or another. I agree that bigger main jets round of all the rough edges, but that doesn't mean two wrongs make a right. Fat jets will cover up bad timing, bad pump shot, wrong idle jets etc.
Good question - I will check the tail pipes. I have pulled all the plugs, and the consistent with the electrodes slightly gray. The rim of the plugs is/are soot - black. .
- First time I have tried "replay with quotes - The jets I changed were the actual nozzles in the barrels and not the pump exhaust valves. You are correct in that as the number designating exhaust hole sized in the pump exhaust valve goes up, more gas is diverted back to the float chamber and the amount of gas injected by the accelerator pump is reduced.
Regarding the tail pipes on the C4 - a dark gray, not brown and not a dark sooty black. They are dirty such that if I touch the inside of the pipe, soot comes off on my finger. I took the C$ out on Sunday and headed up a hill toward the Sierras. At about 60 in 4th I pushed the accelerator to the floor. As expected the acceleration oscillated, but because of hill, the rpm increased slowly. (Normally we would drop a gear or two to really accelerate). After a couple of seconds, I was at 3600 rpm and the oscillation stopped and the acceleration became smooth and strong. At this point, I'm thinking this is where the accelerator pump stopped squirting. Of course this is "seat of the pants", and because I'm thinking the accelerator pump is providing too much fuel, I'm seeing the situation as one supporting that theory. Does any one else out there ever try stuff like this - and see similar results?
I don't know what the US spec is, but the European cars have no hole in the check valve in the bottom of the float chamber. All the fuel is sprayed into the port. If you think there is too much fuel from the accelerator pump you could try some check valves with a bleed. All this is best done on a rolling road with some lambda sensors, and someone with Weber experience running it !
Accelerator pumps won't do that. The pump shot is just so temporary. I can let you borrow two timing lights.
did the carb sync process include the use of a 12 cylinder mercury manometer? with this, you can't realistically sync the throttles and overall linkage operation in an accurate fashion....uni-syn tools wont cut it when it comes to the "crack" the throttle surge in the u-fube column... FWIW
they are NOT cheap $5000 to 10,000.00 USD...NO mercury!(I pd 5K 25 years ago) use triple distilled lab quality... There is a reason that these are so expensive, yes, I could hgave made one-but I would have missed a couple of serious safety apparatus inclusions which fERRARI had engineered/included...think mercury and how lethal it is and the partial pressure law as well as ambint vapor pressure temp! Do you know how dangerous it is just handling this compound? NOT for high school science students that gotta c+ in the mid-term!
I just use a few mercury filled "carb stix". I distinctly remember the Chem 3B hoops we had to jump thorough before signing up to use a fume hood for mercury experiments. There are now safer alternatives. http://www.carbtune.co.uk/ 3 of these will cost you US$294
Yes - I bought a couple sets at the local Honda shop - they used mercury columns to balance the carb on the old 4 cylinder engines. I don't know if they are still available form honda. The mercury column set up sure makes it a bunch easier.
I'm going to quit worrying about this for now, The car runs so good everywhere else in the performance envelope. Maybe I can get a hold of Bill Badurski at some point. Thanks to all Aaron
So Yesterday I was reading an old Road and Track (1970) where they did a Road Test on a Daytona and I came across this phrase "a tendency to stumble if too much gas is dumped in at low speeds". Hmmmm - OK This situation had already spurred me on to do a slightly early service - Checking valve clearances, float levels, Throttle stop... and I checked the advance of the distributors. I'm setting them up with US specifications this time. 13 degrees total advance from weights all the way in to all the way out against the wall of the advance housing at 3500 distributor rpm. (That is 26 degrees at 7000 crank rpm). I get 10 degrees (20 at the crank shaft) of advance in the first 1500 rpm (3000 crank shaft rpm). With this advance curve set, I was planning to set the initial advance at 10 crank shaft degrees. Does this sound OK? Also put a new coat of wrinkle finish on the valve covers - someday I will need to do something to clean up the looks of the headers. A
The RPM band between 2500 and 3500 is the critical band where the advance curve has to be right. The combination of too much gas and too much advance at the same time leads to misfire conditions, as an overly rich mixture is harder to ignite, and you are trying to apply the spark before enough compression has been obtained in the cyclinder. At higher TPMs, turbulence in the mixture and the wave action in the intake system, lead to higher cylinder pressures at the spark timing point. You can check this by taking 2-5 degrees of timing off the spark (crank angle) and see how this influences the stumble. If it does influence the stumble, then try adding a degree of timing until you find the critical point.
